PM / devfreq: fix missing unlock on error in exynos4_busfreq_pm_notifier_event()

Add the missing unlock before return from function
exynos4_busfreq_pm_notifier_event() in the error
handling case.

This issue introduced by commit 8fa938
(PM / devfreq: exynos4_bus: honor RCU lock usage)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
[Resolved path conflicts by MyungJoo Ham]
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>